Participant retention in a continental-scale citizen science project increases with the diversity of species detected Sustaining the efforts of volunteers is a challenge facing citizen science programs. Research on volunteer management shows that a diversity of factors may be correlated with sustained volunteerism. In the present article, we explore retention of participants in a large-scale citizen science project. We focus on Project FeederWatch, a bird-monitoring program. Using data from 17,991 participants, we found that the probability of retention increased with the diversity of species (species richness) reported by a participant, but retention was unrelated to the overall abundance of birds reported. Participants who successfully submitted an observation were more likely to remain in the project the following year (82.0% interannual retention) than people who registered but never submitted an observation (39.7%). Two measures of effort were positively correlated with retention. This work provides a case study for examining how demographic information and scientific data collected by participants can be mined to understand volunteer retention in environmental monitoring projects. David N. Bonter, Victoria Y. Martin, Emma I. Greig, and Tina B. Phillips. BioScience, 2023, 0, 1–8. doi.org/10.1093/biosci/biad041.